This invention relates, in general, to a detector apparatus for detecting submicron particulates in a gaseous carrier; and, in particular, this invention is particularly useful for detecting localized overheating within the core of a gas cooled dynamoelectric machine.
Localized overheating may occur in the stator core of a large gas cooled dynamoelectric machine causing damage to the core which can lead to extensive machine outage and costly repairs. One of the ways in which localized overheating can occur is by damage to the surface of the stator teeth such that electrical contact can take place between core laminations leading to a flow of electric current and resistance heating when the machine is under load. Localized overheating of this nature can generate enough heat to melt the core laminations. It is therefore highly desirable to provide an apparatus for the early detection of localized overheating in a dynamoelectric machine so that the machine load can be reduced and corrective action taken prior to the occurrence of severe damage.
In U.S. Pat. No. 3,573,460 to Skala, issued Apr. 6, 1971, a device is described which will detect the presence of submicron particulates in a gaseous carrier. The patented device comprises an ionization section having a radioactive source applied thereto and a detector section having a voltage source applied thereto. The carrier gas is ionized and a current measurement is taken across the ionized gas flowing between two oppositely charged electrodes to detect the presence of submicron particulates. The electrical conductivity of the ionized gas changes if submicron particulates are entrained in the carrier gas and hence the detected current will decrease because of the collision of ions in the gas with any submicron particulates, since these ions will attach themselves to those particulates and will not contribute to the ion current. The device described in the Skala patent may be generally referred to as an ion chamber detector.
In U.S. Pat. No. 3,427,880 to Grobel et al, issued Feb. 18, 1969, parts of a gas cooled dynamoelectric machine are coated with a selected polymeric material which will decompose at a relatively safe temperature to produce submicron pyrolysate particulates. These pyrolysates may be detected in a device such as the ion chamber detector, and hence, an early advanced warning of localized overheating may be obtained when the ion chamber detector is used in combination with a gas cooled dynamoelectric machine. One example of a coating material is a polyalphamethylstyrene polymer which will commence to give off decomposition particles (pyrolysates) at about 185.degree. C (365.degree. F) in a pressurized hydrogen atmosphere, as measured in one experiment using an ion chamber detector.
Within the environment of a gas cooled dynamoelectric machine, non-pyrolysate particulates may become entrained in the machine gas coolant. Such non-pyrolysate particulates are not due to machine overheating but may, for example, be due to oil mist particles given off from the oil used with the machine shaft seals. While the presence of oil mist particles may be of interest, their presence can adversely affect the desired operation of the ion chamber detector to identify pyrolysates due to localized overheating. The oil mist particles can also pick up ions in the ion chamber detector upon collision with them, thus preventing them from contributing to the ion current and, in a manner similar to pyrolysates, causing a drop in the ion current and a false signal which could lead to a premature or unnecessary machine shutdown. On the other hand, the presence of oil mist vapor in the ion chamber detector can cause erratic readings in the ion chamber detector which can mask a true signal indicative of localized overheating.